The characterisation of fluid-fluid or fluid-solid dispersions, for example with respect to segregational stability and structural stability, as well as the separation behavior in the centrifugal field, is an important task in research, the design of (large) technical separation processes, the development of new products, as well as in quality control close to the production line. The particle size, as well as the distribution of particle sizes, plays a special role here. Ideally, this is to be surveyed without dilution, which means in the original state, because changing the composition can also lead to changes in the measured size (dilution agglomeration, for example).
There are a number of different measurement methods known, which are distinguished with respect to the physical measurement procedure, the area of application (for example, concentration of the sample, range of particle sizes) as well as the measurement options (for example, resolution, type of particle size distribution, measurement accuracy) (Allan, T.: Particle Size Measurement, Kluwer Academic Publishers, Netherlands (1999)/Leschonski, K.: Particle measurement technology, report from the Bunsen Company for Physical Chemistry, (1984)). Regardless of whether fractionated or non-fractionated measurement techniques are involved, all instruments used up until now allow the determination of the particle size of only one sample. In other words, multiple samples must always be measured one after the other. First, this is costly in terms of time because the samples to be measured are often away from the measurement chamber, the chamber must be washed and dried, and the next sample must be poured in. Second, the samples are not measured under identical conditions (for example, temperature drift, subjective and hardware-caused settings particularities, erroneous settings, electronic noise level). Third, a validation of the instruments and/or the measurement for a reference sample is not possible in parallel, which means simultaneously with the actual measurement. In addition, common to all known methods is that various substance parameters (for example, viscosity of the dispersion medium, optical constants) for the samples to be analysed must be known even for diluted samples, in order to be able to calculate a distribution of particle sizes from the measurement results evaluated based on volume. For concentrated dispersions, additional particle-particle interaction effects and particle-fluid interaction effects are to be taken into consideration, such as the increasing substance-specific hydrodynamic interaction (hindrance function) that is non-linear with the volume concentration, for example. This substance characteristic is of importance for high-resolution fractionated measurement methods in particular.
Until now, the sample-specific data had to be compiled through previous tests of the sample both with rheologic and optical measurement methods, and prepared in a suitable manner, for example using special input menus for the respective analysis procedures for the determination of particle sizes. The entire procedural chain is very costly in terms of time, is tainted by measurement errors and cannot be automated. It also proves to be particularly limiting that the predominant majority of the particle size measurement methods can only be used for diluted or even highly-diluted substance samples. For this reason, many micro-dispersions and nano-dispersions cannot be measured under conditions that are close to the product.
Although fractionated measurement methods are distinguished from non-fractionated methods (for example, static or dynamic light scattering) by a significantly higher resolution, in particular for polymodal samples, the previous technical solutions realized are characterised by a series of inadequacies. The following fractionated measurement methods are currently known for dispersions: Gravity sedimentation methods, centrifugal field sedimentation methods (disc centrifuges, photo cuvette centrifuges and manometer centrifuges)
Disc centrifuges are laboratory devices with a sample chamber in the form of a disc, which is accelerated to between 600 and 24,000 revolutions per minute−1 (CPS Instruments, Inc. USA, http://www.cpsinstruments.com/Brookhaven Instruments Corporation USA, http://www.bic.com). When the specified final speed is reached, the suspension to be analysed is introduced onto the surface of a fluid that was poured into the sample chamber in advance. In principle, the sample is quite heavily diluted by doing so. In addition, hydrodynamic instabilities often occur when “immersing” the particles into the spinning fluid. This leads to starting times and starting speeds that are tainted with errors. Since the temperature of the measurement chambers cannot be maintained, a calibration measurement for determining the current base value must first be established by means of reference particles for the density and viscosity of the spinning fluid, which are dependent on the temperature. In principle, this involves the risk of impurity from the displacement of reference particles into the sample to be subsequently applied.
As a result of the centrifugal force, the particles begin to migrate outwards according to their size. A suitable source of radiation is positioned such that radiation is transmitted through the disc at a position determined by the manufacturer, for the most part at the outside edge. Through scattering and absorption, the sedimenting particles reduce the intensity of the radiation, which is measured by a sensor at a constant position. From the time elapsed and the particle migration and the measurement of the attenuated radiation intensity, the size and the concentration of the particles are determined. Disc centrifuges are in the position to detect particles in the range of sizes from 0.01 μm to 40 μm. The sample throughput is limited by the fact that only one sample can be measured in each case.
Known photo cuvette centrifuges likewise measure the opacity of a light beam or laser beam only for one sample and at one level (Shimadzu Scientific Instruments (SSI) North America: http://www.ssi.shimadzu.com/Horiba: http://www.horiba-particle.com/). In contrast to the disc centrifuges, the particles are evenly dispersed in a transparent cuvette at the start of the test. As a result of the centrifugal acceleration, the particles begin to settle and pass the light sensor according to individual size classes. For this reason, the opacity decreases with time, and the particle size distribution can be calculated from this temporal transmission increase and the associated sedimentation velocity.
A measurement of the particle concentration by means of a detection method for electromagnetic radiation forms the basis of both methods, the disc centrifuge and the photo cuvette centrifuge. Corresponding to the prior art, radiation sources are used in the visible range for the determination of the particle size distribution. The extinction coefficients that depend on particle size corresponding to the Mie Theory are required for a calculation of the particle size distribution evaluated on the basis of volume or mass (van de Hulst, H. C.: Light Scattering by Small Particles, Dover Publications Inc., New York (1981)/Kerker, Milton: The scattering of light and other electromagnetic radiation, Academic Press, New York, San Francisco, London, (1967)).
If X-ray radiation is used, absorption coefficients that depend on particle size can be used. However, in addition to the technical radiation safety aspects, this has the disadvantage that only samples with materials of higher atomic numbers (typically>13) can be measured. Biological samples, for example, cannot be analysed for this reason.
In order to test the sedimentation behavior that is dependent on particle sizes for particles in the centrifugal field, a manometer centrifuge can be used, whose principle is based on the measurement of the hydrodynamic difference in pressure between two measurement levels in a sedimentation cell (Beiser, M., Stahl, W.: Influence of Additives on the Sedimentation Behaviour of Fine Grained Solids in the Centrifugal Field, Filtech Europe 2003—Conference Proceedings, Volume I—L-Session, page 1-465-I-472). When a solid substance that has a higher density than the fluid precipitates out, the average mass density of the suspension volume between the two measurement levels decreases continuously, and the hydrodynamic difference in pressure likewise reduces as a result. The process continues until the separation level between the clear liquid and the sedimentation zone has passed the lower measurement level. If all particles settle at the same speed, the difference in hydrodynamic pressure decreases linearly with time. However, if there are particles in the suspension that settle rapidly and slowly, the change in pressure is initially made up of both parts, and if the particles settling more rapidly have left the measurement volume, the slope of the pressure curve changes. If n particle classes are present in the suspension, n−1 inflexion points result in the temporal pressure curve, or a non-linear curve for a continuous particle size distribution. Information about the sedimentation mechanisms can be derived from these pressure curves, i.e. at what concentrations the transition between zone and cluster sedimentation lies, for example. A large disadvantage is the costly measurement of the pressure in the rotating sedimentation cell and the output of the temporal pressure curve during the centrifugation. Even here, only one sample can be tested during a measurement.
In addition, it is common for the technical devices for the previously described centrifugation method to be targeted to the measurement of suspensions. If anything, a modification must be made for the measurement of emulsions. Mixed dispersions (milk products, for example) that exhibit simultaneous flotation and sedimentation segregation in principle could not be analysed with this method with respect to particle size without prior separation.
In the European patent specification EP 0 840 887 B1, a method and a device for the automatic analysis of geometric, mechanical and rheological parameters of substance systems and materials is described, which is based on the different cuvettes or measurement systems matched to the tested commodity and the test parameter(s), which are also placed in different positions radially, being placed on a carriage or rotor positioned horizontally or vertically, and being subjected to a time-variable acceleration that is preset or controlled depending on the course of the process. The change in the local and temporal composition of the substance system induced by the acceleration, the geometric arrangement or position of the materials, or the position of the corresponding sample specimen is detected with high resolution by means of mechanical or electromagnetic waves. Multiple material characteristics—such as sedimentation velocity, flotation velocity, viscosity, viscoelasticity, concentration by volume, distribution of particle sizes, particle types, elasticity, adhesion, adhesiveness or tensile strength—as well as their time dependencies are calculated online or offline simultaneously from these signals using appropriate algorithms.
The subject matter of the patent applications DE 102 08 707.5-52 A1 and EP 1 386 135 A2 is a method and a device, with which both the stability or instability of a dispersion can be measured, or with which stabilising or destabilising effects on a dispersion can be tested. At the same time, the instantaneous measurement of the local composition of the dispersion is made possible with high local and temporal resolution using the overall level of the measurement cells as well as their temporal change in intervals of a hundredth of a second without movement of measurement cell, transmitter or receiver in relation to one another.
Likewise, for the multi-channel devices from the patent specifications EP 0840887 B1, DE 102 08 707.5-52 A1 and EP 1 386 135 A2, the transmission is recorded, solved in terms of location and time without a temperature option (exception patent specifications DE 102 08 707.5-52 A1 and EP 1 386 135 A2) for the samples. Until now, it has proven particularly disadvantageous here that the transmission signal was recorded only as a virtual, device-dependent intensity, and that the method extended primarily to the ascertainment of the particle-free solution/dispersion boundary layer. A conversion of the transmitted intensity into extinction values proportional to concentration was not provided either for dilute or for undiluted dispersions in particular. Also lacking are appropriate mathematical algorithms that make possible the simultaneous experimental determination of the particle sizes of a sample and the substance-specific characteristics (for example, size-related extinction coefficients, hindrance or flux density function that depend on concentration) required for the calculation of these sizes using the multi-channel capability under the same measurement conditions and with the same measurement values. An automatic software-based analysis and documentation of these characteristics was not provided.
An object of the invention is based on the elimination of the disadvantages of the solutions described in the prior art.